I would say you can do this in two way: white hat and so called black hat methods.
Yes that is right, out there is a lot of different tools and it really does not matter which one you use. First you have to make some research, and find out needs and desires of your niche. How? Just look around and go to I think first tool that any marketers use, Google keyword tool, then you find a keywords that are most viewed. Of course it does not mean that these keywords going to be most targeted. These are only examples of high volume traffic with broad keywords, but then use these high volume keywords to find deep keyword lists. Deep keywords and long tail phrases are variations of one root keyword. You can easily find that keyword using different SEO tools.., if you want I can give you some example of free ones..
So going to the point, you should include in your targeted keyword list, keywords that are broad and deep, in that way you will not miss high volume of traffic and your website will be getting traffic from all major sub niches in your market, and you will get deep keywords with less volume but with highly targeted customers.
I suggest you to look also on this free online tool www.keywordindex.com
The other technique that is rather more black hat and it is not exactly about having most targeted keywords, so it is a bit above of your question, but this is about getting targeted customers, so I think it is good to consider it in your research.
I told you that it is going be more from practice. So the point here is to find related topics that are around interests of the particular niche, using demographic and gander research. It is like out of box thinking. So let take an example, if someone looking for a wrestling videos, you can easily find the gender and demographics and then think about what else that demographic looks for in internet, and ad this keyword to your site. This will increase your targeted audience.
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